The P-51D has a fuel relief valve that feeds unused fuel into the left wing tank. So you generally want to use up some fuel out of that tank first to make room for the unused fuel to return to the tank. If the left wing tank is full, the unused fuel will simply be dumped out and wasted. However, if you are carrying fuel in the fuselage tank, it affects aerodynamics significantly and you want to get rid of that first, so once you've made some room in the left wing tank, you would want to switch to the fuselage tank and use all of that fuel before returning to the wing tanks (or drop tanks if those are carried).